Vibration right off idle

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plumkrazee70

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I just finished my 70 Dart with a 340 and 42RH conversion.

I took it out for a test drive, however the vibration is so bad, I had to take it home. I can feel it idling in drive or reverse and continues right off idle. I brought it up to 35 mph and the vibration/noise increased with speed. I took it home, but am un certain where to look.

The driveshaft, is new with joints (wasn't a shortened core, but built) the converter is new and supposedly the trans has been rebuilt (only real unknown) The 8 3/4 is also brand new.

My first thought was the converter is externally balanced, but I didn't see any weights and I used the 340 flexplate. I am afraid to drive it as I don't want to mess something up.
 
What year 340 ? The later 340s were cast cranks with external balance front damper.

Early 340s were internal balanced with forged cranks.

If you have a late 340 with and early internal balance damper on it, that will vibrate.

Here is a 72 340 external balance that uses the counter weighted damper on the front.
Screenshot_20210401-203129_Gallery.jpg
 
What is the 42RH conversion ?

Edit:
OK looked it up on Google 42RH is a Dakota od auto trans.

Want to find out if your 340 is an internal or external balance to begin with. If it is a '70 would think that would be internal balance.

Have to see if that 42RH was set up for internal or external balance from the Dakora it came out of.
 
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It is a '68 so its neutral balanced. I just double checked the converter I got was for internal balance as well. 42RH or A500 Overdrive Transmission.
 
Do you have a Lock Up Converter on that 42RH? Or is the converter non lock up.

If it is a lock up converter that may not be wired up right causing the vibration ??
 
Do you have a Lock Up Converter on that 42RH? Or is the converter non lock up.

If it is a lock up converter that may not be wired up right causing the vibration ??

It is a lockup converter. I didn't think that would matter. I believe the lockup just comes on or it doesn't based on 12v. It's worth a look.
 
Does it vibrate in park with engine rpm or only when the vehicle is moving?
 
Tying to think how that lockup works, could possibly need power to disengage it and no power to engage it.

There are 2 pin or 3 pin wiring plugs on the trans to help identify what it is.
 
This overdrive trans came out of a '90 D150 4x4 it's a 2 pin electric coupler.

20201027_110952.jpg


20201027_111140.jpg
 
Did you get an aftermarket converter for it or did you use the converter that came with the 42RH trans?
 
Tying to think how that lockup works, could possibly need power to disengage it and no power to engage it.

There are 2 pin or 3 pin wiring plugs on the trans to help identify what it is.

Yea it's a 3 pin circuit. The middle pin is 12v and the other two are OD and LU
Did you get an aftermarket converter for it or did you use the converter that came with the 42RH trans?

I just bought a factory lockup converter for a 92 Dakota. I wanted to see how it performed first. The two pin is non lockup.
 
OK, sounds like you have everything right for your setup then.

Just need to figure where the vibration is coming from.

Hope they sent you the proper Lockup Converter to fit your trans.

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I tried to fit the 90 overdrive converter from this pictured trans into my '76 D100 727 trans, no way it would fit, was a neutral balance converter that I needed to go with 90 318. Had to cut the weights off the 727 converter to make it neutral balance then it worked fine.

Mated up the '90 318 Roller engine to the 727 trans.
 
I'm going to try and put the front on ramps and the rear on stands and then run it in gear. Then look to see if I can see any wobble anywhere. I'm not sure what else to do.
 
Check your tranny fluid on the stick on a white paper towel to see that it is nice and bright cherry red yet, and not any fine filings in it.

Just wondering if that new converter is fitting in the front pump right.

Worse scenario is you drop the pan to check for any metal or bearing material, and make sure the filter looks clean yet.

Even before dropping the pan you could unbolt the 4 flexplate bolts, then rotate the converter by hand to make sure it is turning freely.

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So here is the question? Was it a good working used transmission to begin with. Or a rebuildable core that you rebuilt.

Did the trans come with it's own converter and you decided to just get a new one?

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Just trying to figure if it was a good working transmission, what changed.
The New Converter ???
 
Another member mentioned vibration issues with poly mount. Installed a rubber mount, and vibration went away. 42RH torque converter should be neutral balance converter. You need to figure out of your 340 is external balance or internal balance. If its internal balanced a regular A904 flexplate bolts right up
 
So yeah those overdrive transmissions push back and crowd the tunnel, so definitely first check there for clearance problems.
 
If it is not vibrating in neutral or park, then that kind of says the new converter is rolling in the front pump OK.

So looking like more than likely that the tail overdrive unit is rubbing on the tunnel.

I have heard that people actually have to cut open and modifying their A Body tunnels for the Overdrive transmissions cause they fit so tight.
 
Has this engine ever run before? Or is this a 100% new build?

All the trans recommendations make sense, but don't forget the basics. Verify your firing order? We all screw it up at least once and it can cause some odd issues with the right cyls switched.
 
So this has been an 8 year build.

1. The trans was purchased used (supposed to be a good working unit). It did not come with a converter. I purchased one.
2. My 340 is internally balanced, using 904 flexplate
3. I have a rubber trans mount.

So I'm leaning towards something hitting the floor. Or the upper hoop. I'll take a closer look tomorrow after work.
 
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